Plant Physiology 155: 1851-1870 (2011)

Regulation of seed germination in the close Arabidopsis relative Lepidium sativum: A global tissue specific transcript analysis ^[C][W][OA]

Abstract
The completion of germination in Lepidium sativum and other endospermic seeds (e.g. Arabidopsis) is regulated by two opposing forces, the growth potential of the radicle (RAD) and the resistance to this growth from the micropylar endosperm cap (CAP) surrounding it. We show by puncture force measurement that the CAP progressively weakens during germination and we have conducted a timecourse transcript analysis of RAD and CAP tissues throughout this process. We have also used specific inhibitors to investigate the importance of transcription, translation and posttranslation levels of regulation of endosperm weakening in isolated CAPs. Although the impact of inhibiting translation is greater, both transcription and translation are required for completion of endosperm weakening in the whole seed population. The majority of genes expressed during this process occur in both tissues, but where they are uniquely expressed, or significantly differentially expressed between tissues, this relates to the functions of the RAD as growing tissue and the CAP as a regulator of germination through weakening. More detailed analysis showed that putative orthologs of cell-wall remodelling genes are expressed in a complex manner during CAP weakening suggesting distinct roles in the RAD and CAP. Expression patterns are also consistent with the CAP being a receptor for environmental signals influencing germination. Inhibitors of the aspartic, serine, and cysteine proteases reduced the number of isolated CAPs in which weakening developed, and inhibition of the 26S proteasome resulted in its complete cessation. This indicates that targeted protein degradation is a major control point for endosperm weakening.

Financial support: Work was supported by the Biotechnology and Biological Sciences Research Council (Grant BB/E006418/1) to W.F.-S., the Deutsche Forschungsgemeinschaft (grant no. DFG LE720/6) and the Deutscher Akademischer Austauschdienst (grant no. DAAD D/0628197) to G.L.-M., the Wissenschaftliche Gesellschaft Freiburg to G.L.-M. and A.L., the Guangdong Natural Science Fundation (07006658) to X.W., a postdoctoral fellowship of the Deutsche Forschungsgemeinschaft to Ke.M. (grant no. MU3114/1-1) and an Alexander von Humboldt-foundation Research Fellowship to K.O.